Charlotte FC defender Ream captained the US side at this year's World Cup on home soil, where they reached the last 16 before being eliminated by Belgium.
"That's it for me. I'm proud of being part of something for 16 years, but it is time to step away from the national team," the former Fulham and Bolton Wanderers player told reporters ahead of a friendly match against Chile in his hometown of St. Louis, Missouri.
The US, who beat Chile 4-2, next play Mexico in Glendale, Arizona, on Saturday before hosting Canada in St. Paul, Minnesota, on October 6.